The invention relates to an injection moulding device comprising a mould body having a cavity, an elongated nozzle seated in the cavity, a valve pin coaxially in a channel of the nozzle and actuating means connected to the valve pin for axially displacing the valve pin in the nozzle, the valve pin exiting the channel via a bore at the upper end of the nozzle and being guided by a bush in said bore.
A valve gated injection moulding device of the above-mentioned type is known from EP-A-0 836 925 in the name of the applicant. By axial displacement of the valve pin, the gate of the nozzle is opened and closed to control the flow of molten thermoplastic material into a mould cavity. The valve pin head projects via a bore in the manifold body from the upper part of the nozzle and is seated in a sliding bush, which is axially displaced by means of a lever arm. The lever arm is connected to a hydraulic cylinder that is placed at a radially offset location from the valve pin. Leakage of molten thermoplastic material along the bore in the manifold body is prevented by a relatively long bore and by a narrow gap between the wall of the bore and the needle, wherein a gap width of less than 20 micrometers is required. Such narrow gaps are difficult to manufacture and may cause metal-to-metal wear (fretting, corrosion) of the needle against the bush.
It is an object of the present invention to provide a valve gated injection moulding device, having a valve pin which can be guided in a bush without leakage, and without the risk of prohibitive wear.
Hereto the injection moulding device according to the invention is characterized in that cooling means are associated with said bush.
By providing a cooled bush, the viscosity of the molten thermoplastic material in the gap between the needle and the bush may be increased. Hence, leakage may be prevented, even if the gap width is relatively large. Furthermore, the solidified thermoplastic material forms a lubricating material between the bush and the valve pin, such that wear of the needle is prevented.
In a preferred embodiment, the bush projects above a surface of the mould body, the cooling means comprising a cooling plate, spaced away from the mould body, and associated with the upper end of the bush. The cooling plate may form the base on which the actuating cylinder of the valve pin is placed, to provide a thermal insulation, such that the seals and the pressure medium of the cylinder are not exposed to the high temperatures of the manifold. In this way a very compact and leakage free actuating mechanism of the valve pin is obtained. The cooling plate may be supported on the manifold via a refractive spacer member. The bush may be seated in a cavity in the mould body at a distance from the sidewalls of the cavity, the bush having a lower shoulder, a clamping ring being screwed into the cavity and engages with the shoulder on the bush, an inner wall of the clamping ring being spaced away from the sidewall of the bush. In this way, heat transfer from the heated manifold, which may have a temperature of for instance 350xc2x0 C. to the cool cylinder is via the bush minimised. Hereby the internal seals of the cylinder are not exposed to high temperatures, such that the service life of the seals is increased.